1. Field of the Invention
The present invention relates to photomask blanks and photomasks for use in the microfabrication of semiconductor integrated circuits or the like, and methods of manufacturing the same. It also relates to phase shift mask blanks, phase shift masks, and methods of manufacturing the same. More particularly, it relates to halftone phase shift mask blanks and phase shift masks which can attenuate the intensity of exposure wavelength light with a phase shift film, and methods of manufacturing the same.
2. Prior Art
Photomasks are used in a broad range of applications, including the manufacture of semiconductor integrated circuit (IC), large-scale integration (LSI) and VLSI chips. They are basically constructed by starting with a photomask blank comprising a transparent substrate and a light-shielding film made primarily of chromium thereon and processing the light-shielding film by photolithography using UV radiation or electron beams for thereby forming a desired pattern in the film. The market demand for ever higher levels of integration in semiconductor integrated circuits has led to a rapid reduction in the minimum feature size of photomask patterns. Such miniaturization has been achieved in part by the use of shorter wavelength exposure light. Although exposure using shorter wavelength light does improve resolution, it has undesirable effects, such as reducing the focal depth, lowering process stability and adversely impacting product yield.
One pattern transfer technique that has been effective for resolving such problems is phase shifting. This involves the use of a phase shift mask as the mask for transferring microscopic circuit patterns.
As shown in accompanying FIGS. 10A and 10B, a phase shift mask (typically, halftone phase shift mask) is generally composed of a substrate on which a phase shift film has been patterned. The mask has both exposed substrate areas (first light-transmitting areas) “a” on which there is no phase shift film, and phase shifters (second light-transmitting areas) “b” that form a pattern region on the mask. The phase shift mask improves the contrast of a transferred image by providing, as shown in FIG. 10B, a phase difference of 180 degrees between light passing through the pattern region and light passing through the non-pattern region, and utilizing the destructive interference of light at the boundary regions of the pattern to set the light intensity in the areas of interference to zero. The use of phase shifting also makes it possible to increase the focal depth at the necessary resolution. Hence, compared with a conventional mask having an ordinary light-shielding pattern such as chromium film, the phase shift mask can improve resolution and increase the margin of the exposure process.
For practical purposes, such phase shift masks can be broadly categorized, according to the light-transmitting characteristics of the phase shifter, as either completely transmitting phase shift masks or halftone phase shift masks. Completely transmitting phase shift masks are masks in which the phase shifter has the same light transmittance as the substrate, and which are thus transparent to light at the exposure wavelength. In halftone phase shift masks, the phase shifter has a light transmittance that ranges from about several percent to several tens of percent the transmittance of exposed substrate areas.
FIG. 1 shows the basic structure of a halftone phase shift mask blank, and FIG. 2 shows the basic structure of a halftone phase shift mask. The halftone phase shift mask blank shown in FIG. 1 includes a transparent substrate 1 and a halftone phase shift film 2 formed over the substantially entire surface of the substrate 1. The halftone phase shift mask shown in FIG. 2 is arrived at by patterning the phase shift film 2 of the blank and includes phase shifters 2a which form the pattern regions of the mask and exposed substrate areas 1a on which there is no phase shift film. Exposure light that has passed through the phase shifter 2a phase-shifted relative to exposure light that has passed through the exposed substrate area 1a. The transmittance of the phase shifter 2a is selected such that exposure light which has passed through the phase shifter 2a has too low an intensity to sensitize the resist on the substrate to which the pattern is being transferred. Accordingly, the phase shifter 2a functions to substantially shield out the exposure light.
Halftone phase shift masks of the above type encompass halftone phase shift masks of the single-layer type which are simple in structure and easy to manufacture. Single-layer halftone phase shift masks known to the art include those described in JP-A 7-140635 which have a phase shifting film composed of a molybdenum silicide material such as MoSiO or MoSiON.
These phase shift masks are manufactured from phase shift mask blanks. It is important for the phase shift mask blanks to exhibit a distinct etched cross-sectional geometry and low defectiveness during mask pattern formation while satisfying optical properties such as transmittance, reflectance and refractive index to the exposure wavelength of interest.
Phase shift films in such phase shift mask blanks are generally deposited by sputtering. For the deposition, a metal silicide target is typically used which is prepared by mixing a metal and silicon in such a compositional ratio that a desired transmittance is obtainable after deposition, and sintering the mixture. The phase shift film thus deposited has a constant compositional ratio of silicon to metal in a depth direction of the film.
In the aforementioned single-layer halftone phase shift mask, however, the adjustment of optical properties to desired values dictates a certain film composition. It is then difficult to produce a phase shift film that can satisfy other desired properties as well.
To avoid this problem, a phase shift multilayer film has been proposed comprising a plurality of layers including a layer satisfying optical properties and a layer satisfying other properties such as chemical resistance. However, the phase shift film comprising a plurality of layers has a problem of exacerbated line edge roughness because steps are frequently formed in sidewalls of an etched pattern during pattern formation.
Aside from the phase shift films mentioned above, a similar problem arises where a plurality of layers having different compositions are stacked to provide one function, for example, in the case of a reflection type photomask having a reflective film formed by stacking alternate layers of different compositions.
Particularly when a phase shift mask is manufactured from a phase shift mask blank, the phase shift film is typically patterned by reactive ion etching (RIE). When a layer having a constant composition of elements in a depth direction thereof is subjected to RIE, etching proceeds not only in a vertical direction toward the substrate, but also in a lateral direction. Then at the end of etching, the boundary between the etched-away portion and the retained phase shift film portion is inclined in cross section. When exposure is made through a mask having an inclined boundary geometry, the contrast of the mask pattern at the boundary becomes blurred. Only a low contrast is provided upon exposure of a very fine pattern.